The former Russian spy allegedly poisoned for his fierce criticism of his country’s government died last night .

Alexander Litvinenko, 43, died in intensive care at London’s University College Hospital at 9.21pm, despite the medical team doing "everything possible" to save his life.

There had been a major deterioration in his condition overnight on Wednesday, when he suffered a heart attack. The former security agent was placed on a ventilator but failed to recover.

Confirmation of his death - a tragic climax to the spy thriller-style saga of Cold War era espionage - came as Mr Litvinenko’s last, prophetic words were disclosed.

"The bastards got me but they won’t get everybody," he reportedly told his friend and filmmaker, Andrei Nekrasov. "I want to survive, just to show them."

The potential diplomatic ramifications of the Russian’s death were unclear last night.

Scotland Yard’s counter terrorism command, which is leading the investigation into the cause of Mr Litvinenko’s illness and subsequent death, said last night that it was treating his death as "unexplained". It has not launched a murder inquiry.

There was still no confirmation last night of what had had transformed the former Federal Security Service colonel from a fit and healthy 43-year-old into an emaciated, weak and desperately ill man in just three-and-a-half weeks.

The mystery deepened yesterday, after doctors appeared to discount what were suggested as the two most likely causes; thallium poisoning or some kind of radioactivity.

Extensive toxicology tests have already been carried out and there will now be a post mortem examination as doctors and police try to get to the bottom of the mystery. This could take a considerable length of time.

Mr Litvinenko’s wife, Marina, has been at his bedside all week. She has yet to speak publicly about the chain of events that ended in her husband’s death.

However, one of Mr Litvinenko’s closest friends Alex Goldfarb, told the BBC last night: "We are terribly shocked. The family has gone into mourning.

"We are very sorry for Alexander and his family. He died with a clear conscience and a clear heart and with dignity."

Friends of the ex-KGB officer believe he was poisoned by Russian security services because of his fierce criticism of President Vladimir Putin’s regime. However both the Kremlin and Russia’s security agencies have denied any involvement.

A Scotland Yard spokeswoman said inquiries were continuing, adding: "Although formal identification has not taken place at this stage, we are satisfied that the deceased is Mr Litvinenko and the matter is being investigated as an unexplained death."

Police are investigating how he became ill and earlier in the week said that they suspected a "deliberate poisoning".

Investigators are working through potential toxins one by one to see what might have caused Mr Litvinenko’s condition.

Mr Litvinenko, a defector to Britain who was granted asylum and citizenship, is thought to have been poisoned on November 1. He had been investigating the murder of Russian investigative journalist Anna Politkovskaya.

On the day it happened he had two meetings. The first was at a London hotel - named as the Millennium Hotel - where he had tea with a former KGB officer and another Russian.

The second meeting was with Italian academic Mario Scaramella, a former consultant on a commission investigating KGB activities in Italy, at the Itsu sushi restaurant in Piccadilly.

While there, he reportedly helped himself at the restaurant’s buffet and received soup from an attendant.

Mr Litvinenko was subsequently taken ill and later admitted to Barnet General Hospital. He was transferred to University College Hospital last Friday, around the same time as the police became involved.

Mr Litvinenko was being treated under armed guard at the hospital.

He had been in a stable but serious condition but his condition dramatically deteriorated overnight on Wednesday when he suffered a heart attack. He was put on a ventilator and was on sedation, but could not recover.

"But the figure who greeted me looked like a survivor from the Nazi concentration camps."

Doctors remained baffled at what could have caused Mr Litvinenko’s illness but they have ruled out speculation that the heavy metal thallium was to blame and have also said radiation poisoning was unlikely.

Chemistry expert Dr Andrea Sella, of University College London, said: "They have a problem. They have to find some unspecified poison and they don’t know what it is. They don’t know whether it is a single substance or a mixture."

Mr Litvinenko is a former employee of the KGB and later the FSB, one of its successor organisations. The SVR is another successor of the Soviet agency.

The Italian academic who met Mr Litvinenko in a sushi bar on the day he was allegedly poisoned has said they discussed documents he had received which included "alarming" facts that left both men fearing for their lives.

Mario Scaramella, a former consultant on the Italian government’s Mitrokhin Commission, which investigated the KGB’s activities in Italy, suggested that Mr Litvinenko could have been targeted for his own work for the commission.

Vladimir Putin was head of the FSB when Mr Litvinenko, then a colonel in the organisation, publicly voiced concerns about deep-rooted corruption in the service.

Mr Litvinenko was sacked, arrested, and charged with corruption, before fleeing to the UK where he successfully claimed political asylum.